C2000 Ramp Generator - Generate ramp output

Library

Embedded Coder/ Embedded Targets/ Processors/ Texas Instruments C2000/ Optimization/ C28x DMC

Description

This block generates ramp output (out) from the slope of the ramp signal (gain), DC offset in the ramp signal (offset), and frequency of the ramp signal (freq) inputs. All of the inputs and output are 32-bit fixed-point numbers with Q values between 1 and 29.

Algorithm

The block's output (out) at the sampling instant k is governed by the following algorithm:

out(k) = angle(k) * gain(k) + offset(k)

For out(k) > 1, out(k) = out(k) - 1. For out(k) < -1, out(k) = out(k) + 1.

Angle(k) is defined as follows:

angle(k) = angle(k-1) + freq(k) * Maximum step angle
for angle(k) > 1, angle(k) = angle(k) - 1
for angle(k) < -1, angle(k) = angle(k) + 1

The frequency of the ramp output is controlled by a precision frequency generation algorithm that relies on the modulo nature of the finite length variables. The frequency of the output ramp signal is equal to

f = (Maximum step angle * sampling rate) / 2^m

where m represents the fractional length of the data type of the inputs.

All math operations are carried out in fixed-point arithmetic, where the fixed-point fractional length is determined by the block's inputs.

Note To generate optimized code from this block, enable the TI C28x or TI C28x (ISO) Code Replacement Library. See About Code Replacement Libraries and Optimization for Embedded TargetsDesktop Targets.

Dialog Box

Maximum step angle

The maximum step size, which determines the rate of change of the output (i.e., the minimum period of the ramp signal).

When you enter double-precision floating-point values for parameters in the IQ Math blocks, the software converts them to single-precision values that are compatible with the behavior on c28x processor.

Examples

The following model demonstrates the Ramp Generator block. The Constant and Scope blocks are available in Simulink Commonly Used Blocks.

In your model, select Simulation > Configuration Parameters. On the Solver pane, set Type to Fixed-step and Solver to Discrete (no continuous states). Set the parameter values for the blocks as shown in the following table.

Block	Connects to	Parameter	Value
Constant	Ramp Generator - `gain`	`Constant value` `Sample time` `Output data type` `Output scalig value`	`1` `0.001` `sfix(32)` `2^-9`
Constant	Ramp Generator - `offset`	`Constant value` `Sample time` `Output data type` `Output scalig value`	`0` `inf` `sfix(32)` `2^-9`
Constant	Ramp Generator - `freq`	`Constant value` `Sample time` `Output data type` `Output scalig value`	`0.001` `inf` `sfix(32)` `2^-9`
C2000 Ramp Generator	Scope and Floating Scope (Simulink block)	`Maximum step angle`	`1`

When you run the model, the Scope block generates the following output (drag a zoom box around a portion of the output to change the display).

With fixed point calculations in IQMath, for a given frequency input on the block, f_input, the equation is:

f = (Maximum step angle * f_input * sampling rate) / 2^m

For example, if f_input = 0.001, the real value, 1, counts as fixed point with a fractional length of 9:

f = (1 * 1 * (1/0.001) ) / 2⁹ = 1.9531 Hz

Where 0.001 is the block sample time.

If we use normal math, and f_input is a non-fixed point real value, then:

f = (Maximum step angle * f_input * sampling rate) / 1

For example, if we are using floating point calculation:

f = (1 * 0.001 * (1/0.001) ) / 1 = 1 Hz

When using fixed point with fractional length 9, the expected period becomes:

T = 1/f = 1/1.9531 Hz = 0.5120 s

This result is what the above Scope output shows.

Note If you use different fractional lengths for the fixed point calculations, the output frequency varies depending on the precision.

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